Recently, it has been predicted that fossil fuels would be depleted in future. In order to solve energy problems, it is an urgent need to establish technologies for producing next-generation energy alterative to fossil fuels. As one of them, a technology for biofuel production using photosynthetic organisms such as cyanobacteria and algae has attracted attention and been researched. The photosynthetic organisms can produce biofuels from carbon, which was photosynthetically fixed from CO2 and water by using light as an energy source. In addition, the photosynthetic organisms are not competitive with food raw materials and can realize carbon-neutral fuel production. Because of these advantages, the photosynthetic organisms are expected as a next-generation energy production system.
Cyanobacteria (also known as blue-green algae) belong to a group of eubacteria and have an ability to fix CO2 and produce oxygen through photosynthesis. Cyanobacteria, which have an outer membrane and a cell wall formed of peptidoglycan, fall into the category of gram-negative bacteria but are phylogenetically far from typical gram-negative bacteria. More than billion years ago, cyanobacteria were engulfed by eukaryotic cells. Such intracellular symbiont (primary symbiosis), cyanobacteria, are considered as an origin of chloroplasts. Thus cyanobacteria have been widely used in photosynthesis studies as an ancestor organism of chloroplasts.
Cyanobacteria grow fast, have a high photosynthetic ability, and have a transformation ability. Because of this, cyanobacterial cells, to which foreign DNA is introduced, can be used in microbiological production of substances and thus have attracted attention as a microbial host for producing a biofuel. As examples of biofuels produced by cyanobacteria, hydrogen (Non-Patent Literature 1), ethanol (Non-Patent Literature 2), isobutanol (Non-Patent Literature 3) and fatty acids (Non-Patent Literature 4) have been reported. Non-Patent Literature 4 and Patent Literature 1 describe a method for converting inorganic carbon to a fatty acid by culturing a recombinant cyanobacterial cell producing exogenic acyl-ACP thioesterase. Patent Literature 2 describes that the amount of carbon assimilation per cell volume is increased by inducing functional loss of a gene for an AbrB-like transcriptional regulator (cyAbrB) in a cyanobacterium.
Meanwhile, Non-Patent Literature 5 reports that an AbrB-like transcriptional regulator, Sll0822, of Synechocystis sp. PCC6803, is essential for activating transcription of a gene involved in nitrogen metabolism, and that, the expression levels of genes involved in fatty acid biosynthesis became equal to or lower than those of a wild-type strain in a strain having a deletion of gene sll0822. Non-Patent Literature 6 suggests that Sll0822 serves as a repressor of a gene involved in an inorganic carbon uptake system in high CO2 conditions, and presumably serves as a factor involved in regulating C/N balance in cells.
(Patent Literature 1) JP-A-2011-505838
(Patent Literature 2) JP-A-2011-229482
(Non-Patent Literature 1) Yoshino F. et al. (2007) Mar. Biotechnol. 9: 101-112
(Non-Patent Literature 2) Deng M. D. and Coleman J. R. (1999) Appl. Environ Microbiol. 65: 523-528
(Non-Patent Literature 3) Atsumi S. et al. (2009) Nat. Biotechnol. 27: 1177-1180
(Non-Patent Literature 4) Liu X. et al. (2011) Proc. Natl. Acad. Sci. USA. 108: 6899-6904
(Non-Patent Literature 5) Ishii A. and Hihara Y. (2008) Plant Physiol. 148: 660-670
(Non-Patent Literature 6) Lieman-Hurwitz J. et al. (2009) Environ Microbiol. 11: 927-936